The structural and vibrational properties of bismuth selenide (Bi 2 Se 3 ) have been studied by means of x-ray diffraction and Raman scattering measurements up to 20 and 30 GPa, respectively. The measurements have been complemented with ab initio total-energy and lattice dynamics calculations. Our experimental results evidence a phase transition from the low-pressure rhombohedral (R-3m) phase (α-Bi 2 Se 3 ) with sixfold coordination for Bi to a monoclinic C2/m structure (β-Bi 2 Se 3 ) with sevenfold coordination for Bi above 10 GPa. The equation of state and the pressure dependence of the lattice parameters and volume of α and β phases of Bi 2 Se 3 are reported. Furthermore, the presence of a pressure-induced electronic topological phase transition in α-Bi 2 Se 3 is discussed. Raman measurements evidence that Bi 2 Se 3 undergoes two additional phase transitions around 20 and 28 GPa, likely toward a monoclinic C2/c and a disordered body-centered cubic structure with 8-fold and 9-or 10-fold coordination, respectively. These two high-pressure structures are the same as those recently found at high pressures in Bi 2 Te 3 and Sb 2 Te 3 . On pressure release, Bi 2 Se 3 reverts to the original rhombohedral phase after considerable hysteresis. Symmetries, frequencies, and pressure coefficients of the Raman and infrared modes in the different phases are reported and discussed.

We report an experimental and theoretical lattice dynamics study of antimony telluride (Sb 2 Te 3 ) up to 26 GPa together with a theoretical study of its structural stability under pressure. Raman-active modes of the low-pressure rhombohedral (R-3m) phase were observed up to 7.7 GPa. Changes of the frequencies and linewidths were observed around 3.5 GPa where an electronic topological transition was previously found. Raman mode changes evidence phase transitions at 7.7, 14.5, and 25 GPa. The frequencies and pressure coefficients of the new phases above 7.7 and 14.5 GPa agree with those calculated for the monoclinic C2/m and C2/c structures recently observed at high pressures in Bi 2 Te 3 , and also for the C2/m phase in the case of Bi 2 Se 3 and Sb 2 Te 3 . Above 25 GPa no Raman-active modes are observed in Sb 2 Te 3 similarly to the case of Bi 2 Te 3 and Bi 2 Se 3 . Therefore, it is possible that the structure of Sb 2 Te 3 above 25 GPa is the same disordered bcc phase already found in Bi 2 Te 3 by x-ray diffraction studies. Upon pressure release, Sb 2 Te 3 reverts back to the original rhombohedral phase after considerable hysteresis. Raman-and IR-mode symmetries, frequencies and pressure coefficients in the different phases are reported and discussed.

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High pressure structural stability of BaLiF3 J. Appl. Phys. 110, 123505 (2011) Pressure effects on the transitions between disordered phases in supercooled liquid silicon J. Chem. Phys. 135, 204508 (2011) Microfabrication of controlled-geometry samples for the laser-heated diamond-anvil cell using focused ion beam technology Rev. Sci. Instrum. 82, 115106 (2011) First-principles investigations of elastic stability and electronic structure of cubic platinum carbide under pressure J. Appl. Phys. 110, 103507 (2011) Additional information on J. Appl. Phys. High-pressure optical absorption and Raman scattering measurements have been performed in defect chalcopyrite (DC) CdGa 2 Se 4 up to 22 GPa during two pressure cycles to investigate the pressure-induced order-disorder phase transitions taking place in this ordered-vacancy compound. Our measurements reveal that on decreasing pressure from 22 GPa, the sample does not revert to the initial phase but likely to a disordered zinc blende (DZ) structure the direct bandgap and Raman-active modes of which have been measured during a second upstroke. Our measurements have been complemented with electronic structure and lattice dynamical ab initio calculations. Lattice dynamical calculations have helped us to discuss and assign the symmetries of the Raman modes of the DC phase. Additionally, our electronic band structure calculations have helped us in discussing the order-disorder effects taking place above 6-8 GPa during the first upstroke.

Spectrum of -Bi 2 Te 3 at 13.3 GPa and the corresponding fit of Voigt profiles corresponding to the Raman-active modes of the C2/m structure.

We have studied BaWO 4 under compression at room temperature by means of x-ray diffraction and Raman spectroscopy. When compressed with neon as pressuretransmitting medium (quasi-hydrostatic conditions), we found that BaWO 4 transforms from its low-pressure tetragonal structure into a much denser monoclinic structure. This result confirms our previous theoretical prediction based on ab initio calculations that the scheelite to BaWO 4 -II transition occurs at room temperature if kinetic barriers are suppressed by pressure. However, our experiment without any pressure-transmitting medium has resulted in a phase transition to a completely different structure, suggesting non-hydrostaticity may be responsible for previously reported rich polymorphism in BaWO 4 . The crystal structure of the low-and high-pressure phases from the quasihydrostatic experiments has been Rietveld refined. Additionally, for the tetragonal phase the effects of pressure on the unit-cell volume and lattice parameters is discussed.2 Finally, the pressure evolution of the Raman modes of different phases is reported and compared with previous studies.

We report a joint experimental and theoretical study of the structural and vibrational properties of synthetic sphaerobismoite (β-Bi 2 O 3 ) at high pressures in which roomtemperature angle-dispersive X-ray diffraction (XRD) and Raman scattering measurements have been complemented with ab initio total-energy and lattice dynamics calculations. Striking changes in Raman spectra were observed around 2 GPa, whereas X-ray diffraction measurements evidence no change in the tetragonal symmetry of the compound up to 20 GPa; however, a significant change exists in the compressibility when increasing pressure above 2 GPa. These features have been understood by means of theoretical calculations, which show that β-Bi 2 O 3 undergoes a pressure-induced isostructural phase transition near 2 GPa. In the new isostructural β′ phase, the Bi 3+ and O 2− environments become more regular than those in the original β phase because of the strong decrease in the activity of the lone electron pair of Bi above 2 GPa. Raman measurements and theoretical calculations provide evidence of the second-order nature of the pressure-induced isostructural transition. Above 20 GPa, XRD measurements suggest a partial amorphization of the sample despite Raman measurements still show weak peaks, probably related to a new unknown phase, which remains up to 27 GPa. On pressure release, XRD patterns and Raman spectra below 2 GPa correspond to elemental Bi−I, thus evidencing a pressure-induced decomposition of the sample during downstroke.

GPa in MgWO 4 . The high-pressure phase has been tentatively assigned to a triclinic structure similar to that of CuWO 4 . We also report and discuss the Raman symmetries, frequencies, and pressure coefficients in the low-and high-pressure phases. In addition, the Raman frequencies for different wolframites are compared and the variation of the mode frequency with the reduced mass across the family is investigated. Finally, the accuracy of theoretical calculations is systematically discussed for MgWO 4 , MnWO 4 , FeWO 4 , CoWO 4 , NiWO 4 , ZnWO 4 , and CdWO 4 .